hey adam, thanks for the check-in! no progress here. i had to slide out to EU for some gigs, got back yesterday.

alas, i think i need a break from "intensive troubleshooting type DIY" for some "instantly gratifying module works type DIY". gotta recharge my batteries. hopefully will pick this up at some point soon.....its alot harder to bail on a module once the front panel is made, though.....

Looking at a dead VCO myself. Have another, working one just next to it for comparison, but damned if I can find the difference. I've swapped out ICs, checked for shorts, polarity of diodes, everything. Wondering if I have to consider replacing the FET or (hopefully not!) the matched transistors.

How could I check if the FET or any of the transistors are bad before ripping them out?_________________Esoteric drones and nonlinear distortion
Custom/handmade experimental instruments
macumbista.net

Assuming no shorts, all chips are getting +/-V correctly and assuming it is not oscillating, check around the core. Don't bother swapping out chips or other parts until you have exhausted all other possibilities.

With the power off, use a multimeter to check every component is connected to the places it should be. Do this on the component side not the solder side. If you actually probe the legs of the components and the pins of the chips, rather than the solder or pads, you can be sure they are connected.

The VCO core is circled in red, check all these parts, don't worry about the waveshaping stuff for now.

Equally important what FET are you using? There is a great variety in FET pinouts and this is a common source of 'nothing happening' in VCOs

Thanks, Andrew. The FET is 2N4391 in both boards. Orientation is the same, one works and one doesn't. So that's not the issue.

I'm wondering if the FET or the transistors are bad somehow. A student did the soldering so I don't know if they got overheated, or static damaged in the case of the 3140 as all the ICs are socketed. Is the FET also static sensitive?

I'm about at the stage where everything else has been checked. But good advice to check from component side. Will report back._________________Esoteric drones and nonlinear distortion
Custom/handmade experimental instruments
macumbista.net

At a certain point, it was easier just to touch each solder joint with the iron to make sure it was good. In the process I found a bad connection to the TL072 at the lowest right hand side of the PCB. All good now. Thanks for your thoughts!_________________Esoteric drones and nonlinear distortion
Custom/handmade experimental instruments
macumbista.net

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